1. Field of the Invention
The present invention relates to a vehicle network system for communications between a plurality of devices mounted on a vehicle.
2. Description of the Related Art
In recent years, a number of electronic devices are mounted for various operations on a vehicle. This makes it necessary to mount a plurality of ECUs (Electronic Control Units) on the vehicle for controlling the actions of those electronic devices. For the cooperative actions of the plural ECUs, the plural ECUs have to share information, and the vehicle is desired that they are connected with each other through a network. FIG. 4 shows a schematic diagram of a vehicle network system in the related art. In FIG. 4, reference numeral 100 designates ECUs mounted on a (not-shown) vehicle, and four ECUs 100 are connected through communication lines 101. In FIG. 4A, the constitution is made of a cascade type of a network system, in which the four ECUs 100 are connected in series. The data can be transmitted from a source ECU 100 to a target ECU 100 through another ECU 100. On the other hand, FIG. 4B shows the constitution of a star network system, in which the communication line 101 is branched at a branching point 102 into four branch lines individually connected with the ECUs 100. In the star network system, the data transmitted from a single ECU 100 can be received by all the remaining ECUs 100.
In JP-A-2001-352336, a network is constituted to a plurality of network nodes which is connected with a star node. The star node is provided with a plurality of interfaces for connecting the network nodes individually. The star nodes release the interface in response to a pilot signal from each network node thereby to transmit a message to at least one network node. This network is provided at the star node with a decision circuit for deciding one interface to be opened for transmitting the message, in case two or more pilot signals arrive simultaneously at the interface of the star node, so that it may solve the conflict of accesses.
In the case of the cascade type of the network system shown in FIG. 4A, however, the ECUs 100 other than the ECUs 100 at the two ends have to receive the data transmitted from the ECU 100 of one side and to transmit the data to the ECU 100 of the other side. As a result, the transmitting circuits and the receiving circuits have to be mounted individually by two, thus causing a problem of a high cost. In the case of the star network system, as shown in FIG. 4B, each ECU 100 may have one transmitting circuit and one receiving circuit. However, metal cables are frequently used as communication lines 101. In case the metal cables are branched, as shown, at a branching point 102, reflection waves are caused by the impedances of the metal cables when signals for the data transmissions are propagated through the communication lines 101. Depending upon the degrees of those reflection waves, there arises a problem that the communication troubles such as the so-called ringing phenomenon may occur.
In recent years, it has been noted that the optical fibers are used as the communication lines constituting the network. In case the communications are performed through the optical fibers, it is advantageous that the ringing phenomenon is not caused by the aforementioned influences of the reflected waves so that the communications can be performed easily and reliably. In the case of the network system mounted on the vehicle, however, the communication lines have to be arranged in the limited space of the vehicle. This may make it necessary to bend and arrange the communication lines. The optical fibers have a problem that they are hard to bend and arrange, i.e., that the optical fibers have a low flexibility for arrangement. In addition, the branching of a light causes a drop of an optical power. As the number of devices to be connected with one optical fiber increases the more, it may be impossible to retain the sufficient S/N ratios in the individual devices.